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Each year it happens - we’re reminded to get our annual flu shot. But why? We're taking a deep-dive into the vaccine's composition to explore how it changes annually and exactly how it protects you.


September 17, 2018

The arrival of fall brings many things: cooler days, pumpkin spice coffee, and flu shots. While you may not automatically associate autumn with influenza, the sooner you get vaccinated for the upcoming flu season, the better your chances of not falling ill. The Centers for Disease Control and Prevention (CDC) recommends that everyone six months of age and older - with rare exception - get a flu shot every season, which can begin as early as October and end as late as May in the United States.1

While the symptoms are sometimes similar to a cold, influenza - aka the flu - is a more severe, viral illness that can lead to additional complications and even hospitalization – with the CDC estimating that flu-related hospitalizations have ranged from 140,000 to 710,000 since 2010.1

Recent studies show that the flu vaccine reduces the risk of flu illness by 40 to 60 percent among the United States' population during seasons when the vaccine is well-matched with circulating viruses.With those figures, an annual flu vaccine remains one of the best defense methods against catching and spreading seasonal flu.1

Why Do I Need to Get the Flu Shot Annually?

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According to the CDC, the flu vaccine is recommended every season for two reasons1:

  • Your body's immune response from vaccination – it declines over time, making an annual vaccine necessary for optimum protection.
  • Constantly changing flu viruses - the formulation of the flu virus is reviewed each year and sometimes updated to keep up with the changing flu viruses.

The influenza viruses, or Candidate Vaccine Viruses (CVVs), are selected based on what research indicates will be most common during the upcoming season. The CVVS are what the CDC or another public health partner prepare for vaccine manufactures to use to produce a flu vaccine.6

A flu shot does not take effect immediately, which is why it's recommended that you get vaccinated before flu season even begins. Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. The antibodies are what provide protection against infection with the viruses that are in the vaccine.1

People with severe, life-threatening allergies to the flu vaccine or any ingredient in the vaccine are among those who should NOT receive an annual flu shot.3 Individuals with egg allergies are advised to consult their physician regarding the flu vaccine. Those with milder reactions should be able to receive any licensed, age-appropriate flu shot.3

It's also important to note that certain flu shots have different age indications. For example, it's recommended that people younger than 65 years of age not get the high-dose flu shot or the flu shot with adjuvant, while people who are younger than 18 years old or older than 64 years old are advised not to get the intradermal flu shot.3

Most flu shots are designed with a specific age group in mind due to the weakening of our immune defenses as we get older. The high-dose flu shot, for instance, contains four times the amount of antigen, which prompts the body to make antibodies, than a regular flu shot.4

How Many Types or Strains of the Flu Are There?

There are three types of flu viruses that can affect humans: A, B, and C. A fourth type, D viruses, primarily affect cattle and are not known to infect or cause illness in people.

Type A and B cause the seasonal influenza epidemics that you typically hear referenced in news reports. Type C, which an annual vaccine does not protect against, also causes the flu, but the symptoms tend to be much less severe.5

Type A

Wild birds commonly act as hosts for this flu virus, also known as Avian or Bird Flu, which does not normally infect humans. However, human influenza A viruses are common and constantly changing. These viruses are divided into subtypes based on two proteins on the surface of the virus - the hemagglutinin (H) and the neuraminidase (N), and they can further be broken down into different strains.5 Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2) viruses, according to the CDC.

The emergence of a new type A virus can cause an influenza pandemic, which is a global outbreak of a new influenza A virus. The last outbreak occurred in the spring of 2009 when a different H1N1 virus began circulating; it was the first influenza pandemic in more than 40 years.5

Type B

Unlike type A flu viruses, type B is only found in humans. These viruses are not classified by subtype and do not cause pandemics, which is a disease that has spread across a large geographic area. They generally cause a less severe reaction than type A, but can still be harmful.5

Type C

These viruses are also only found in people; however, they tend to be milder than type A and B. People generally do not become very ill from type C flu viruses.5

Different strains of the flu virus mutate over time and replace the older strains of the virus. Regularly altering flu viruses are one of the reasons why it's important to get a flu shot each year.5

Traditional flu vaccines (“trivalent” vaccines) are made to protect against three flu viruses: two subtypes of influenza A viruses (H1N1 and H3N2) and an influenza B virus. There are also flu vaccines made to protect against four flu viruses (“quadrivalent” vaccines), which protect against an additional B virus.1

How Exactly Are Flu Vaccines Made?

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The following three influenza vaccine production technologies have received approval from the U.S. Food and Drug Administration (FDA)6:

  • egg-based flu vaccine,
  • cell-based flu vaccine,
  • recombinant flu vaccine.

Egg-Based Flu Vaccines

This vaccine production method is the most common and has been used for more than 70 years.Egg-based vaccine manufacturing is used to make both inactivated (killed) vaccine, which is usually called the “flu shot," and live attenuated (weakened) vaccine, which is known as the “nasal spray."6

For the shot and nasal spray, the CVVs are injected into fertilized hen’s eggs that are then incubated for several days to allow the viruses to replicate.6

The process then differs for the two types of vaccines. For flu shots, the viruses are inactivated or killed, meaning they no longer contain active CVVs. Meanwhile, the viruses are weakened for the nasal spray and do not cause severe symptoms often associated with influenza illness.1

The FDA tests and approves vaccines prior to release and shipment.

Cell-Based Flu Vaccines

Until recently, this production process, which received FDA-approval in 2012, also began with egg-grown CVVs per FDA regulations. In August 2016, the FDA issued an approval for Seqirus, the sole FDA-approved cell-based flu vaccine manufacturer in the United States, to use cell-grown CVVs.6

While the production process is similar to the egg-based manufacturing method, a significant difference is that the influenza viruses used in the cell-based vaccine are isolated and grown in cultured cells of mammals rather than in hens’ eggs.7

Cell-based flu vaccine production does not require chicken eggs because the vaccine viruses used to make vaccine are grown in animal cells.7

Recombinant Flu Vaccines

The third production method, which was approved for use in 2013, involves the use of recombinant technology, and is currently the only 100 percent egg-free vaccine on the U.S. market.6

Recombinant technology, or recombinant DNA technology, involves the joining of DNA molecules from two different species that are inserted into a host organism to produce new genetic combinations.

In the case of the seasonal flu vaccine, manufacturers isolate a certain protein from a naturally occurring recommended vaccine virus (the HA protein, which induces an immune response in people). These proteins are then combined with portions of another virus that grows well in insect cells.6

This “recombinant” vaccine virus is then mixed with insect cells and allowed to replicate. The flu HA protein is then harvested from these cells and purified. The purified protein is packaged while waiting for FDA testing and approval to release lots.6

At this time, Flublok is the only influenza vaccine produced using recombinant technology that has received FDA-approval for use in the U.S.6

Aside from the manufacturing process itself, the main difference between the three is production time. Egg-based vaccines take the longest to produce because of their dependence on egg supply. Cell-based vaccines are slightly quicker to produce, but recombinant vaccines take the least amount of time because they do not rely on egg supply and are not limited by the selection of vaccine viruses that are adapted from growth in eggs.6

What Vaccine Is Right for Me?

You may be experiencing information overload at this point and wondering, "Which influenza vaccine is the one for me?" The CDC's simple answer: A licensed, recommended, and age-appropriate vaccine.

According to the CDC, there is no preference for one vaccine over another among each season's recommended, approved influenza vaccines.8 Just as the components of the vaccine change annually, so do the recommended products.

For instance, the CDC recommended during the 2017-2018 season that only injectable influenza vaccines be used. The organization advised against the use of the nasal spray flu vaccine due to concerns about its effectiveness.1

If you find it difficult to sort through the information each year on your own, don't worry. Schedule an appointment or just walk in to your neighborhood MedExpress center today to discuss the best option for you.


1 CDC: Influenza (Flu): Key Facts About Seasonal Flu Vaccine. Last updated October 30, 2017. Accessed May 8, 2018.

2 CDC: Influenza (Flu): Vaccine Effectiveness - How Well Does the Flu Vaccine Work? Last updated October 3, 2017. Accessed May 8, 2018.

3 CDC: Influenza (Flu) Vaccination: Who Should Do It, Who Should Not and Who Should Take Precautions. Last updated October 3, 2017. Accessed May 8, 2018.

4 CDC: Influenza (Flu): Fluzone High-Dose Seasonal Influenza Vaccine. Last updated December 14, 2017. Accessed August 6, 2018.

5 CDC: Influenza (Flu): Types of Influenza Viruses. Last updated September 27, 2017. Accessed August 6, 2018.

6 CDC: Influenza (Flu): How Influenza (Flu) Vaccines Are Made. Last updated November 16, 2016. Accessed May 8, 2018.

7 CDC: Influenza (Flu): Cell-Based Flu Vaccines. Last updated November 7, 2016. Accessed May 8, 2018.

8 CDC: Influenza (Flu): Seasonal Flu Shot. Last updated June 14, 2018. Accessed August 13, 2018.

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